The invention relates to a hydraulic brake booster for a vehicle brake apparatus having a pressurizing apparatus with a pump and a pressurized reservoir and further including a reservoir switchover piston adapted to be subjected to the reservoir pressure and engagable with a control valve. In such a structure, the control valve is in proximity to a booster cylinder and arranged to control both a connection of the pressurizing apparatus with the booster cylinder and a connection of the booster cylinder with a relief means. A brake booster apparatus such as denoted above is already well-known.
In such brake boosters, the problem arises that when the supply of pressure medium is intact, only a small engaging stroke at the control valve is required, which is solely in order to switch over the control valve. However, when the supply of pressure medium fails, certain usually expensive expedients are necessary in order to uncouple the control valve from the booster piston, so that the mechanical drive of the pedal force which then must necessarily be transmitted through the main cylinder piston can be accomplished.